1. Field of the Invention
The present invention pertains to a radio-frequency modulator and, more particularly, to an acousto-radio frequency modulator.
2. Description of the Related Art
In the laser modulation arena a standard tool for frequency shifting is called the acousto-optic modulator (“AOM”) or more specifically acousto-optic frequency shifter as AOM's can be used for a variety of functions including not only frequency shift but fast beam displacement over multiple physical communication channels and amplitude modulation. For instance, AOMs can shift a light beam by a specific frequency through Brillouin scattering and do so by only the sum or difference of the modulation frequency with no inter-modulation products. The modulating frequency is that of an acoustic wave transmitted through a transparent medium which a passing laser beam interacts with, acquiring a component of the medium's movement as a Doppler shift added to or subtracted from that of the laser.
No such device exists for radio frequencies (“RF”). RF applications instead use what are referred to as “diode mixers”, which are non-linear, for their fast switching frequencies. If an RF modulator were available, “mixing” of RF and audio frequencies could take place without the complexities associated therewith. Square-wave on/off RF switching driven by a local oscillator is what gives an RF mixer its frequency components as the Fourier transform of a square wave is made up of a series of harmonic elements reconstructing that waveform. Among these Fourier components are the desired sum and difference as well as a host of other undesirables known as intermodulation (“IM”) products.
A great deal of innovation, complexity and cost is dedicated to suppressing those IM products through various diode balancing circuit techniques and filters. Typically in RF applications the difference between RF and local oscillator (“LO”) produced by the mixers is used as the intermediate frequency (“IF”) at which information extraction takes place at manageable frequencies. If a RF modulator were available, however, it may not be as useful as an RF mixer in non-typical applications because acoustic wave frequencies, in AOM materials at least, tend not to exceed several gigahertz in current practice. Thus reaching a 60 MHz or even 500 MHz IF from, say 35 GHz, is far outside the reach of today's performance.
One area in which these issues are important is RADAR (“radio detection and ranging”). In World War II, the British developed and utilized systems for remotely sensing the relative position of incoming planes of the German Luftwaffe. RADAR uses radio frequency (“RF”) electromagnetic waves to detect and locate objects at great distances even in bad weather or in total darkness. More particularly, a RADAR system broadcasts RF waves into a field of view, and objects in the field of view reflect the RF waves back to the RADAR system. The characteristics of the reflected waves (i.e., amplitude, phase, etc.) can then be interpreted to determine the position and velocity of the object that reflected the RF wave.
Typical RADAR designs fielded today are composed of subsystems which, but for a few exceptions are, on their own, not necessarily expensive. As integrated units in a single radar system, total cost multiplies due to difficulties in producibility, i.e., the ability to reliably manufacture the system to meet performance specifications. Subsystem and system level yields are low enough due to parts failure, handling or integration followed by test and retest, that final per system costs far exceed the summation of subsystems.
Several efforts attempting to gauge parts count and cost under conventional radar design paradigms with the intent to simplify the device through a reduction in parts have been undertaken. Reducing parts count obviously reduces cost. But, more importantly, parts reduction enhances producibility through the potential for miniaturization or microwave/millimeter wave integrated circuit (“MIMIC”) designs that may occupy a single board thus reducing integration and test associated with fewer subsystems.
Conventional designs, however, have proved marginal in several ways. The number of parts remains generally unchanged regardless of organization, inhibiting (though not precluding) efforts to fit everything on a single board. Variation in circuit organization raises concerns for performance, notably diode mixer intermodulation products of multiple input RF tones on a single channel with their capacity to blind the receiver to what is target signal and what are simply byproducts of the mixing process. Unfortunately, regardless of circuit layout, conventional design efforts resulted in conventional problems with no significant breakthrough in parts reduction.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.